1. Field of the Invention
The present invention relates to methods and apparatus for extruding cellular products, and especially for extruding on an advancing filamentary core, as, for example, an electrical conductor wire, a covering of plastic insulation which can be selectively extruded in various configurations such as, for example, a solid plastic covering, a foamed plastic covering or a combined foamed plastic inner section and a more dense plastic outer section covering.
2. Description of the Prior Art
In the manufacture of plastic insulated wires, it is often desirable to provide wire having either a foamed plastic insulating sheathing or a uniform dual plastic insulating sheathing coaxial with the wire. For example, in dual insulated wire the sheathing can comprise an inner layer of foamed plastic for providing the primary insulating covering, and a solid plastic outer layer for providing both additional insulation and a protective covering against abrasion and moisture for both the inner layer and the wire core.
Extruders for forming insulated plastic coverings upon filamentary cores, as, for example, an electrical wire, which term will be used hereinafter and in the annexed claims to include any form of filamentary core, are well known and have been described in U.S. Pat. Nos. 2,766,481 and 2,911,676, issued to G. E. Henning, and U.S. Pat. Nos. 2,928,130 and 3,078,514, issued to A. N. Gray. Such extruders generally include an extrusion cylinder and an extrusion head, the extrusion cylinder having a stock screw rotatably mounted within the bore of the cylinder for forcing a plastic compound from an entrance end of the bore to a delivery end thereof while simultaneously working the plastic compound into a viscous fluent state. The extrusion head is mounted at the delivery end of the cylinder and includes both an extrusion chamber for delivering the fluent plastic compound to an extrusion die at the exit end of the chamber, and a core tube which is centrally positioned in the chamber in axial alignment with and spaced from the die for guiding the advancing wire into and through the center of the die during the extrusion process.
One method of extruding uniform coverings of cellular plastics upon wires is disclosed in U.S. Pat. Nos. 2,766,481, issued to G. E. Henning, and 3,068,532, issued to W. T. Higgins. Therein, a prepared granular composition comprising polyethylene resin and a heat-decomposable blowing agent is continuously fed into the entrance end of the bore of an extrusion cylinder. As the composition is worked and moved through the cylinder by the stock screw, the blowing agent decomposes but is prevented by the extruder design from substantially expanding the composition until the composition is passed out of the die as a covering on the wire.
The use of a blowing agent for extruding an expandable plastic gives rise to an inherent problem, that being the plating out, in the forming portion of the die, of the heat-decomposable blowing agent admixed with the plastic, which requires the extrusion pressure to be increased to achieve a constant flow rate. Furthermore, the residue formed in the die eventually leads to eccentricity of the sheath and out-of-roundness of the product.
A second method and apparatus for extruding a single uniform covering of cellular plastic on a wire comprises forcing a gas, under pressure, into the plastic mass in the extrusion cylinder at a point sufficiently removed from the discharge end of the cylinder. The gas is so introduced as to make it reasonably certain that the gas will be thoroughly and uniformly mixed with the plastic mass by the time the mixture passes the discharge end of the stock screw. Extruders using such arrangement have been described in U.S. Pat. Nos. 2,848,739, issued to G. E. Henning; 2,928,130, issued to A. N. Gray; and 3,251,911, issued to R. A. Hansen. In the Henning patent, the gas is introduced through a porous material and into a portion of the plastic mass diverted by a counter-rotating screw, centrally located within the main stock screw, before the plastic mass is recirculated. Although the second method avoids the use of a heat-decomposable blowing agent and the resulting plating-out problem, an inherent problem with the second method is that the extruded foamed insulation does not always have a uniform bubble distribution throughout. This non-uniformity occurs from the surging found in screw devices which produces a slight pumping effect in the delivery of the plastic composition to the die area.
U.S. Pat. No. 3,466,705, issued to C. A. Richie discloses apparatus for forming foamable plastic materials in a tubular shape for use as packing material to protect fragile devices by combining the first and second methods described above when taken in their broadest sense. Here, a standard extrusion cylinder and stock screw delivers a mixed foamable plastic material to an extrusion die which includes a body member having a central bore, an outwardly curved expansion guide member axially aligned with the body member, and a hollow cylindrical mandrel spaced from and coaxially mounted within the bore of the body member to extend beyond both the body member and the guide member. The foamable material is extruded around the mandrel and allowed to expand within the die and guide member to form a cellular plastic pipe. When a lower density cellular plastic is desired, a foam-augmenting fluid is forced radially outwardly from the mandrel and/or radially inwardly from the guide member into the already expanding plastic material to further expand the extrusion product. The Richie apparatus, however, would not appear suitable for adaptation to the manufacturing of communication wire or cable, since the plating-out and surging problems are still inherent therein. Additionally the introduction of a foam-augmenting fluid basically reduces the ability to control the process and obtain uniform bubble size and distribution throughout the extruded material. Furthermore, in a conventional foamed wire insulation extruder described above, the foam-augmenting fluid would have to be applied after the covering has been extruded and is expanding to achieve a comparable process.
When it is desired to extrude two concentric insulating sheaths of plastic material around a wire, several problems have arisen, especially when the inner sheath is an expanded or foam-type plastic, as for example, expanded polyethylene or polypropylene.
One method of extruding a dual insulation covering on a wire is disclosed, for example, in U.S. Pat. No. 3,404,432, issued to F. T. White et al. There a tandem arrangement of extruders is utilized, one extruder for each sheath to be applied to the wire. While this is the simplest method of applying dual insulation, it is difficult to control the eccentricity of successively applied coverings, thus giving rise to nonuniform capacitance variations coaxially in the cross-sectional plane of the sheathed wire. Additionally, where one of the sheaths is an extruded expandable plastic wherein a heat-decomposable blowing agent was used, the plating-out problem discussed above is also inherent to the system.
An improved prior art method of extruding dual concentric plastic sheaths onto an axially advancing conductor wire is disclosed in U.S. Pat. Nos. 3,538,547, issued to M. J. Drabb, and 3,737,490, issued to P. Nicholson. Here, a single extrusion chamber is utilized in which first and second spaced and coaxially aligned dies pass the wire sequentially. Plastic material for the inner sheath is delivered from a first extrusion bore to the first die where the plastic material is applied to and formed around the wire. The insulated wire is thereafter passed through the second die, to which the second plastic material is delivered from a second extrusion bore to impinge upon and be formed around the insulated wire.
The two-die method permits higher line speeds than the tandem method, but requires a critical coaxial alignment of the dies to prevent eccentricities in the insulating sheaths. The most serious defect found in any two-die method is that the pressures in the dies are cumulative, making it difficult to control the blowing of the expanded plastic.
In another prior art method, as disclosed in U.S. Pat. Nos. 3,229,012 and 3,446,883, issued to O. G. Garner, the plastic material for the outer sheath of a dual insulated wire is bled around the entrance portion of the forming die in the extrusion passageway before the insulating material for the first sheath is compressed around the advancing wire such that the second plastic forms a barrier between the forming die and the first sheath.
This method would appear to prevent plating out of the blowing agent where the inner sheath is an expanded plastic and the outer sheath is a solid plastic, but, since the plastic materials for both sheaths communicate with one another directly before the flow of at least the plastic for the inner sheath is permitted to develop, irregularities and nonuniform skin thickness can result.
There exists, therefore, a need for a method and apparatus for extruding a concentric, dual-section, plastic covering onto an elongated wire moving at high line speeds wherein plating out of a blowing agent for expanded plastics is eliminated, or minimized; better control of expansion is obtained; and the inner and outer sections of the covering are concentric and of uniform thickness. Furthermore, since prior art methods and apparatus for extruding dual coverings require a separate extrusion cylinder for each sheath, there also exists a need for a simple and inexpensive method and apparatus which can selectively extrude different configurations of sheathing such as a solid plastic sheath; an expanded plastic sheath; or a concentric, dual-section covering, where the inner section is of an expanded plastic material, from a single source of supply.